DEVELOPING THE XP-86

by Larry Davis

STRAIGHT WING XP-86

In 1939, the first operational airplane powered by a gas turbine engine was built. All previous powered airplanes had either inline or rotary cylinder engine, using either gasoline or diesel for fuel, and driving a propeller. Very simply, the gas turbine used its exhaust gas as power, commonly called thrust. In Nazi Germany, Ernst Heinkel had installed a gas turbine engine in his He-178 airframe, making its first flight on 27 August 1939. Across the Channel in England, British engineer Frank Whittle had one of his new gas turbines installed in a Gloster E.38-29 airframe. These two aircraft would ultimately lead to an entirely new concept which would revolutionize the aviation world - the jet aircraft.

In the US, little concern was paid to the new powerplant. The War Department was busy simply trying to catch up with the rest of the world regarding 'normal' airplane types. There was a war in Europe, and another heating up in the Pacific. And the US was far behind all the nations already involved with regards to high performance, propeller-driven aircraft. It wouldn't be until the development of the Lockheed P-38 and Republic P-47, that the US could expend money and energy on 'frivolous' things like gas turbines and jet aircraft. By that time it would be almost too late.

When the P-51 Mustang (considered the best fighter in World War 2) was developed, the Germans already had an operational jet fighter ready to take on the Allies The Messerschmidt Me-262. The Me-262 was flying and ready for operations as early as 1943. Only stupidity on Hitler's part, and luck by the Allies, kept the German jets from decimating the Allied bomber forces. The Me-262 was a 150 mph faster than any propeller-driven airplane flying.

By 1942, a few US companies were becoming intrigued by the gas turbine. With engines borrowed from Whittle, Bell Aircraft Company constructed the XP-59 Airacomet, which made its first flight on 1 October 1942. But its performance was hardly earthshaking. It wasn't even on a par with contemporary prop fighter types. Lockheed also built a jet fighter, the XP-80. The XP-80 was powered by a DeHavilland Goblin turbine, and made its first ffight on 8 January 1944. Although the XP-80 broke the 500 mph barrier, it still was not competitive with German jets. Republic's XP-84 promised still greater performance, but it wouldn't fly until 1946.

North American Aviation (NAA) was also working on a gas turbine project. North American had experimented in a variety of different directions in an effort to increase the performance of the Mustang, including lighter weight, rocket assists, and jets. They installed a rocket in the aftercooler area of a P-51D. The rocket-assisted P-51D could attain a speed of 515 mph, but only for about a minute. Thoughts were given to a very radical P-51 with both a prop and a jet engine. This design concept had wings that were 'swept' forward. It was found that 'aft sweep' gave wingtip stall problems, so forward sweep was studied first. But the swept-wing P-51 never went beyond the drawing board as the wing suffered from a twisting divergence under load.

North American's engineers saw the future was in the alljet powered aircraft. On 24 August 1944, Ed Horkey talked with Ira Abbott about airfoil selection for a new jet-propelled, high-speed fighter design. They concurred that very thin wings would be required to avoid compressibility shock. Mr. Horkey was concerned about the lift and drag characteristics of such thin wings, and was informed that no recent data existed for any type of wing section with such small thickness ratios.

On 22 November 1944, NAA initiated a design study (RD-265) for a jet fighter proposal. The design was straight forward in all respects, and used a lot of P-51 technology. It was just powered by a gas turbine. The flying surfaces, the wings and tail, were very similar to those found on the latest P-51 designs. The wing was of the latest laminar flow design, with straight leading and trailing edges. There were no'devices' added to the wings to either smooth air flow or increase lift.

The fuselage was short, rotund, and very smooth. The nose was open to induct air to the TG 180 gas turbine engine. The TG 180 was a General Electric license-built version of the DeHavilland Goblin gas turbine. The main problem was that the project (NA-134) was designed FOR THE NAVY. The Navy promptly ordered three prototypes of the NA-134 on 1 January 1945.

The US Army Air Force got interested in the North American Navy jet project in the Spring of 1945. On 18 May 1945, NAA received a letter contract to build three prototype aircraft for the Army, with the designation XP-86. The General Operational Requirements called for a day fighter of medium range, that could operate in both the escort and fighter-bomber missions, with a top speed in excess of 600 mph. This last item in the GOR was considered by many to be outside the scope of any jet designs at the time.

The XP-86 differed considerably from its Navy cousin, now designated the XFJ-1. North American refined the fuselage shape and deleted many of the things designed to aid in low speed performance that were required for aircraft carrier landings. The XP-86 wing had the same planform as the XFJ-1, but the airfoil was much thinner. On both the upper and lower wing seurfaces were the dive brakes, borrowed directly from the A-36A version of the Mustang. The fuselage had a much higher fineness ratio than the XFJ-1, and the intake was oval in shape. Power was the same for both aircraft - the GE TG 180 (J35). The TG 180 had an eleven stage, axial flow compressor, and offered 4,000 lbs of thrust.

The XP-86 would have a pressurized cockpit, hydraulic elevator and aileron boost, and had wingtip fuel tanks that could be jettisoned in emergencies. Armament was the standard for Army Air Force aircraft - six .50 calibre M3 machine guns in the nose, with 267 rounds per gun. The gunsight was the A-1 type, with a AN/APG-5 radar range finder. Under the wings, a pair of pylons could hold up to 2,000 lbs of bombs, drop tanks, or eight 5" HVAR rockets.

The XP-86 was 35.5 feet in length, 13.2 feet in height, with a wingspan of 58.2 feet. With a maximum gross weight of 14,600 lbs. the TG 180-powered XP-86 was estimated to have a rate of climb of 5520'/min, a combat range of 1500 miles, a ferry range of 2240 miles when 170 gallon wingtip tanks were installed, and a service ceiling of 44,900'. But the speed was estimated to be only 582 mph at 10,000' - far below the GOR requlrement of 600+ mph.

On 20 June 1945, the mockup of the XP-86 was unveiled at the North American plant in Inglewood, California. Very sleek in its gloss Pearl Grey paint, the XP-86 mockup was quickly approved by Army Air Force officials. Photos of the mockup show that the aft section of the fuselage had its engine break about midway through the wing root chord. The fuselage was much sleeker than the rotund XFJ-1.

But North American officials knew that the lack of speed would eventually kill the XP-86 program unless something drastic took place, such as a much more powerful gas turbine or some way to reduce the drag encountered at speeds over 500 mph. It did! For many years, the advantages of sweeping the leading edge of the wing to reduce drag rise had been known. But the disadvantages of the swept wing were many, not the least of which was the loss of stability, especially at low speeds. The Germans had encountered this with the Me-262, which had a slight sweep to the outer wing leading edge. They found that by using a movable leading edge surface, commonly referred to as a 'slat', that many of the low speed stability problems could be overcome.

On 14 August 1945, North American Aviation received a research and development grant to develop a swept wing for the XP-86. Two weeks later a .23 scale model of a swept wing XP-86 was built. On 18 September 1945, the XP-86 model was wind tunnel tested. The results were exactly what North American and Army Air Force had been looking for. The swept wing lowered the drag rise and compressibility enough that it brought the XP-86 into the 600+ mph range, even using available gas turbine technology. And the leading edge slats appeared to solve any low speed stability problems encountered with the use of a swept wing. On 1 November 1945, Army Air Force approved the 'new' swept wing version of the XP-86. And the rest is, as they say, HISTORY!


No portion of this article may be used or reprinted without permission from the President of the F-86 Sabre Pilots Association or the editor of Sabre Jet Classics magazine.


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